You should be able to put the 4K MKV files on a flash drive and plug that into your Oppo and play the files directly. I watch the 4K versions that way (I plug the flash drive directly into my TV, but the principle is the same).
Thank you!! I tried this (for the first time on my Oppo, never used the USB port on it before) and it worked great! I was amazed how easy it was to navigate the USB drive (an external 1 TB drive that powers via just the USB cable which made it super convenient). I was literally stunned at the incredible quality of 4K77 and 4K83 (both with DNR/2160p). I have the Despecialized versions that I haven’t unpacked and plan to compare the quality but I can’t imagine it could be better than the 4K versions. I do appreciate all the work that went into the Despecialized versions also so I do intend to check it out too.